1. Field of the Invention
The invention relates generally to the detection of hazardous atmospheric turbulence, and more particularly to the detection of aircraft wake vortices, microbursts, and tornadoes near the ground, and Clear Air Turbulence (CAT) at high altitude.
2. Description of the Prior Art
Hazardous Turbulence Detection: Clear air turbulence, microbursts, aircraft wake vortices, gust fronts, thunderstorms, updrafts, downdrafts, and convective flows all present severe hazards to aircraft. Effects on aircraft encountering any of these hazards range from severe buffeting to the ultimate catastrophe. A system for detecting such hazards is disclosed in U.S. Pat. No. 6,480,142 issued to W. L. Rubin. Hazardous turbulence is detected with the utilization of Doppler shifted frequencies of received radar signals backscattered from sound generated by atmospheric turbulent flows. The radiated radar signals are generally within the UHF or microwave bands. Resulting Doppler frequency bandwidths are used to establish the parameters of the sound emitted by the, hazardous turbulence. Radar reflections are strongest when the acoustic wavelength of the sound is one half the radar wavelength (Bragg effect). The corresponding acoustic frequencies that maximize reflection from UHF and microwave radar fall between 1 kHz and 20 kHz. As will be shown, turbulence generated sound is strongest in the infrasound band below 20 Hz and decreases approximately 6 dB per octave above 50 Hz. Consequently, acoustic reflections are weak for UHF and microwave radar, resulting in short detection ranges.
Wake Vortex Detection: Aircraft generate multiple invisible vortices in their wake as a consequence of lift. The vortices quickly combine to form a pair of primary vortices that are separated by three-quarters of a wingspan. The primary vortices descend after formation and diverge as they approach a height above the ground approximately equal to one-half of their initial separation. Upon reaching this height, the vortices move in opposite directions in calm winds at a speed of 1 to 2 meters per second. A strong crosswind moves both vortices in the same direction at slightly different speeds.
Vortices produced by large and heavy aircraft are hazardous to smaller aircraft since they can induce rolling moments that exceed the smaller aircraft's roll control authority, causing a dangerous loss of altitude, and possible structural failure. Vortex encounters are especially hazardous in the terminal area where light aircraft follow heavy aircraft on the same glide slope and where recovery from an upset may not be possible because of low aircraft altitude.
To minimize wake vortex encounters, the Federal Aviation Administration (FAA) imposes 3 to 6 mile spacing between leader and follower aircraft based on relative aircraft weight categories. Because a strong crosswind moves wake vortices out of the glide path fairly quickly, FAA spacings are overly conservative most of the time. The FAA has projected airline savings of billions of dollars per year if mandated spacings could be safely reduced. Systems have been proposed to adjust aircraft spacing based on vortex duration in the glide slope. Such systems require a real-time vortex sensor.
A Ground Wind Vortex Sensing System (GWVSS) has been developed to determine vortex duration. This system utilizes a row of pole-mounted anemometers perpendicular to the arrival glide slope for detecting the air flow generated by wake vortices as they move away from the glide slope. Each anemometer is mechanically connected to a DC generator, the voltage of which is proportional to the rotational rate of the anemometer. This output voltage is coupled to a graph-like display in which anemometer positions are located along the bottom of the display. A data point above each anemometer position is deflected upward by an amount proportional to the amplitude of the corresponding generator output voltage. A curve is passed through the vertically deflected points. The location of vortices with respect to individual anemometers can be deduced from the shape of the curve. GWVSS, however, is not operationally deployed because poles are not permitted near airport glide slopes for safety reasons.
Another wake vortex detector of the prior art is a horizontally pointing Radar Acoustic Sensing System (RASS). Such a system has detected, tracked, and measured the strength of wake vortices under all weather conditions. RASS radiates overlapping radar and acoustic beams. Because the air's index of refraction is a function of density, the RASS acoustic beam, which consists of a spatial pattern of condensations and rarefactions, generates in situ refractive index variations. Radar waves reflect from these index variations. As previously stated, radar reflections are strongest when the acoustic wavelength is half the radar wavelength. The reflections focus back onto the radar antenna because the radar and acoustic beams have overlapping spherical wavefronts. Radar reflections are Doppler shifted by an amount corresponding to the instantaneous speed of sound. Because a vortex's circular flow speeds up and slows down different parts of a RASS acoustic wave as it passes through the vortex, the radar Doppler spectrum is a mapping of the vortex's line-of-sight velocity distribution. Vortex circulation (vorticity) is deduced from the Doppler spectrum. RASS, however, emits a loud annoying sound, is costly, and expensive to maintain.
Wake vortices radiate audible sounds. These sounds have been recorded 6 and 12 seconds after aircraft passage by a ground-based, vertically pointing 64-element microphone array. A system, having the acronym SOCRATES (Sensor for Optically Characterizing Ring-Eddy Atmospheric Turbulence Emanating Sound), for detecting this sound utilizes a laser-based, opto-acoustic technique to emulate a large array of microphones. SOCRATES has detected sound generated by wake vortices, but not consistently.
Microburst Detection: A microburst is a small-scale, short-lived, intense downdraft produced by cool dense air descending from the base of a convective cloud. When the downdraft reaches the ground, it generates an outburst of damaging wind flow. Microbursts are invisible and can occur anywhere. Nicrobursts typically last less than 10 minutes and are hazardous to aircraft during takeoff and landing.
Winds traveling outward from the touchdown area have an inverted mushroom shape about 5 miles in diameter and 300 to 500 feet high. Outward wind speeds can be as high as 168 mph and vary with height above the ground. There are two types of microbursts—wet and dry. The distinguishing feature between the two is the prevailing environment in which they are produced.
A system, in the prior art, positioned on board an aircraft for detecting microbursts that measures aircraft ground speed and compares it to the air speed of the aircraft is disclosed in U.S. Patent issued to L. M. Green, et al. The difference in speeds is an indication of wind conditions about the aircraft. This system, however, determines the existence of a microburst in the vicinity of the aircraft and does not provide a timely waning for the avoidance of the microburst.
Another method in the prior art, LLWAS (Low Level Windshear Alert System), consists of a centrally located wind sensor and about 12 additional wind sensors located around the airport. A software algorithm detects and measures wind shear based on wind measurements. LLWAS, however, has poor spatial resolution along the arrival glide slope and departure corridor, thereby allowing many microbursts to escape detection.
Still another system in the prior art is the Terminal Doppler Weather Radar (TDWR) a ground based system which detects microburst wind shear in the vicinity of an airport. TDWR, however, detects only radial wind shear relative to its location.
Additionally, its horizontal antenna beams receive strong ground clutter return that reduces wind shear detectability and decreases measurement accuracy. Further, TDWR detects only wet microbursts, it cannot detect dry microbursts.
Tornado Detection: A tornado is a funnel-shaped, swirling turbulent flow having a top that is connected to an overhead thunderstorm cloud and a bottom that is in contact with the ground. Tornados have internal winds that range from 40 to 300 miles per hour and diameters near the funnel bottom that are typically 1000 feet. A tornado travels at an average speed of 25 to 40 miles per hour over an average path length of 4 miles and emits sound over a wide band of frequencies including very low frequency sounds known as infrasound. Tornado damage results from the high internal wind velocities and wind-blown debris. Tornadoes are nearly invisible, marked only by swirling debris at the base of the funnel.
The National Weather Service utilizes radar to identify conditions conducive to the formation of a tornado and issues tornado watches and warnings, which typically average less than 20 minutes, over the broadcast media. This radar detects about 50% of the tornadoes in the Rocly Mountain high plains and about 25% of the tornadoes in the high plains. The relatively low detection rate results from the fact that multiple radar signatures must be simultaneously identified in order to detect atmospheric conditions conducive to tornado formation.
Radar resolution limits the specificity of tornado warnings to several hundred square miles. As a result, National Weather Service warnings mostly serve to heighten people's awareness in the affected area to look for visual and aural signs of an approaching tornado. In these circumstances the chance to reach shelter may have already passed by the time a tornado is sighted.
Other tornado warning devices rely on the detection of electromagnetic, pressure, and acoustic signatures. Electromagnetic signatures are generated by the thunderstorm spawning the tornado. Since all thunderstorms do not generate tornadoes, this method has a high false alarm rate. Devices utilizing low barometric pressure also have high false alarm rates since low pressure may result from an approaching storm having moderate winds. Both methods provide short warning times and no information about tornado distance.
Numerous tornado warning systems have been devised that rely on detecting sound generated by tornadoes. One such system, disclosed in U.S. Pat. No. 5,355,350 issued to H. E. Bass, et al, monitors outdoor noise between 180 and 420 Hz. When the monitored sound intensity exceeds established thresholds, logic is applied to determine whether the level is increasing at a rate indicative of an approaching tornado and therefore whether an alarm should be sounded. This system, because sound in the selected passband is significantly attenuated by the atmosphere, detects tornadoes to a distance of only one-half mile and consequently, provides warning times of only 30 to 60 seconds. This method also does not provide range to the tornado.
Another system for providing tornado warnings is disclosed in U.S. Pat. No. 6,097,296, issued to S. Garza, et al. In this system a narrow pass band acoustic filter is provided which is centered at one Hertz and designed to attenuate signals that are greater than and less than one hertz. In essence the filter has a near zero bandwidth. The wave emanating from this filter is clipped both at the peaks and valleys to obtain a wave that is substantially square. The square wave is coupled to a counter wherein the number of square wave cycles are counted. An alarm sounds when the counter reaches a predetermined number of cycles.
This system is very difficult to implement. A filter that passes only one frequency, if possible, is extremely difficult to design. Further, a filter centered at 1 Hz may not be optimum. The shape of a tornado's infrasound spectrum depends on internal wind speeds and tornado size, giving rise to infrasounds over a bandwidth that may not include one Hertz or may peak at some other frequency. Should the system be operable as described, it would be of limited utility since it does not provide range to the tornado and a concomitant warning time.
CAT Detection: CAT is produced by high altitude atmospheric flows having speeds of 100 to 200 knots. CAT is often hundreds of miles long, tens of miles wide, and thousands of feet high and cannot be seen or avoided by pilots without prior knowledge of its location. NASA has estimated that airlines encounter CAT about 9 times a month, resulting in 24 injuries, principally to flight attendants. Costs due to CAT exceed $100 million a year. These costs include aircraft repair and downtime, flight attendant injuries and time lost, and passenger injuries.
Radar cannot detect CAT because it is composed of clear air which does not contain reflecting aerosols. A system known as LIAR, operating in the intra band, is currently being assessed for its ability to detect CAT. An experimental LIAR system, however, has generated CAT warnings of only a few seconds. As presently configured, LIAR's high cost, size, and weight, in addition to its poor performance to date, make it unattractive for onboard CAT detection.